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Configuring BFD for Layer 2 VPN and VPLS

 

The following procedure describes how to configure Bidirectional Forwarding Detection (BFD) for Layer 2 VPN and VPLS. For VPNs, you configure the BFD sessions on the interfaces carrying traffic from the PE routers to the CE routers.

The BFD protocol is a simple hello mechanism that detects failures in a network. Hello packets are sent at a specified, regular interval. A neighbor failure is detected when the routing device stops receiving a reply after a specified interval. BFD works with a wide variety of network environments and topologies. The failure detection timers for BFD have shorter time limits than default failure detection mechanisms for BGP, so they provide faster detection.

The BFD failure detection timers are adaptive and can be adjusted to be faster or slower. The lower the BFD failure detection timer value, the faster the failure detection and vice versa. For example, the timers can adapt to a higher value if the adjacency fails (that is, the timer detects failures more slowly). Or a neighbor can negotiate a higher value for a timer than the configured value. The timers adapt to a higher value when a BFD session flap occurs more than three times in a span of 15 seconds. A back-off algorithm increases the receive interval by two if the local BFD instance is the reason for the session flap. The transmission interval is increased by two if the remote BFD instance is the reason for the session flap. You can use the clear bfd adaptation command to return BFD interval timers to their configured values. The clear bfd adaptation command is hitless, meaning that the command does not affect traffic flow on the routing device.

  1. You can enable BFD failure detection. The BFD failure detection timers are adaptive and can be adjusted to be faster or slower. The lower the BFD failure detection timer value, the faster the failure detection and vice versa. For example, the timers can adapt to a higher value if the adjacency fails (that is, the timer detects failures more slowly). Or a neighbor can negotiate a higher value for a timer than the configured value. The timers adapt to a higher value when a BFD session flap occurs more than three times in a span of 15 seconds. A back-off algorithm increases the receive (Rx) interval by two if the local BFD instance is the reason for the session flap. The transmission (Tx) interval is increased by two if the remote BFD instance is the reason for the session flap.

    To enable BFD failure detection and specify the threshold for the adaptation of the BFD session detection time, specify a time in milliseconds using the threshold statement. When the detection time adapts to a value equal to or greater than the threshold, a single trap and a single system log message are sent.

    Note

    The threshold time must be equal to or greater than the value specified in the minimum-interval or the minimum-receive-interval statement.

    You can use the clear bfd adaptation command to return BFD interval timers to their configured values. The clear bfd adaptation command is hitless, meaning that the command does not affect traffic flow on the routing device.

  2. You can specify the minimum interval after which the local routing device transmits hello packets and then expects to receive a reply from a neighbor with which it has established a BFD session. You specify the interval in milliseconds using the minimum-interval statement.

    Optionally, instead of using this statement, you can specify the minimum transmit and receive intervals separately using the minimum-interval (specified under the transmit-interval statement) and minimum-receive-interval statements.

  3. You can configure the minimum interval after which the local routing device must receive a reply from a neighbor with which it has established a BFD session. Specify the number of milliseconds using the minimum-receive-interval statement.
  4. You can specify that an interface be declared down when a certain number of hello packets have not been received from a neighboring router through that interface. Specify the number of hello packets by including the multiplier statement.
  5. You can configure BFD sessions not to adapt to changing network conditions by including the no-adaptation statement. We recommend that you do not disable BFD adaptation unless it is preferable to have BFD adaptation disabled in your network.
  6. Specify the transmit interval options for bfd-liveness-detection statement by including the transmit-interval statement. The negotiated transmit interval for a peer is the interval between the sending of BFD packets to peers. The receive interval for a peer is the minimum time that it requires between packets sent from its peer; the receive interval is not negotiated between peers. To determine the transmit interval, each peer compares its configured minimum transmit interval with its peer's minimum receive interval. The larger of the two numbers is accepted as the transmit interval for that peer.

    The transmit-interval statement specifies how often BFD statements are transmitted and includes the following options:

    • minimum-interval milliseconds—Specify the minimum interval in milliseconds at which the local routing device transmits hello packets to a neighbor with which it has established a BFD session.

    • threshold milliseconds—Specify the threshold for the adaptation of the BFD session transmit interval. When the transmit interval adapts to a value greater than the threshold, a single trap and a single system message are sent.

      Note

      The threshold value specified in the threshold statement must be greater than the value specified in the minimum-interval statement for the transmit-interval statement.

  7. Specify the BFD version by including the version statement. You can set BFD to version 1 or allow BFD to determine what version it needs to be by including the automatic option.